Multi Carrier Code Division Multiple Access and OFDM CDMA

	Multi-Carrier CDMA (MC-CDMA)
1993	Prof. Jean-Paul Linnartz started his research on Multi Carrier - Code Division Multiple Access (MC-CDMA) in 1992 at the Department of Electrical Engineering and Computer Sciences, University of California at Berkeley. The research results were first presented to a wide public in 1993 at the PIMRC Conference in Yokohama, Japan. Other researchers were also working on similar schemes at the same time. Different names have been proposed for the transmission method, e.g., OFDM - CDMA but our name MC-CDMA is the most widely adopted.
	Currently, the many papers on the topic of MC-CDMA are presented at every communication conference. The original 1993 paper: (scanned pdf of 2.8 mbytes, pdf)

BACKGROUND

The research on MC-CDMA intends to improve the reliability and performance of wireless radio links. The mobile or indoor radio channel is characterized by 'multipath reception': The signal offered to the receiver contains not only a direct line-of-sight radio wave, but also a large number of reflected radio waves. These reflected waves interfere with the direct wave, which causes significant degradation of the performance of the network. A wireless network has to be designed in such way that the adverse effect of these reflections is minimized. Another critical design objective is high spectrum efficiency. The latter should ensure that the network can accommodate as many users possible within a given frequency band.

The effects of (multipath) radio propagation, modulation, and coding and signal processing techniques on the spectrum efficiency and performance of wireless radio networks are studied, in particular Orthogonal Frequency Division Multiplexing (OFDM) and related transmission methods.

Most conventional modulation techniques are sensitive to intersymbol interference unless the channel symbol rate is small compared to the delay spread of the channel. OFDM is significantly less sensitive to intersymbol interference, because a special set of signals is used to build the composite transmitted signal. The basic idea is that each bit occupies a frequency-time window which ensures little or no distortion of the waveform. In practice it means that bits are transmitted in parallel over a number of frequency nonselective channels. This technique is for instance used in digital audio broadcasting (DAB).

What is MC-CDMA?
Learn more about OFDM
What are the advantages of MC-CDMA?
How can it be implemented?
Some papers

WHAT IS ORTHOGONAL MULTI-CARRIER CDMA?

There are many equivalent ways to describe MC-CDMA:

MC-CDMA is a form of CDMA or spread spectrum, but we apply the spreading in the frequency domain (rather than in the time domain as in Direct Sequence CDMA).
MC-CDMA is a form of Direct Sequence CDMA, but after spreading, a Fourier Transform (FFT) is performed.
MC-CDMA is a form of Orthogonal Frequency Division Multiplexing (OFDM), but we first apply an orthogonal matrix operation to the user bits. Therefor, MC-CDMA is sometimes also called "CDMA-OFDM".
MC-CDMA is a form of Direct Sequence CDMA, but our code sequence is the Fourier Transform of a Walsh Hadamard sequence.
MC-CDMA is a form of frequency diversity. Each bit is transmitted simultaneously (in parallel) on many different subcarriers. Each subcarrier has a (constant) phase offset. The set of frequency offsets form a code to distinguish different users.

P.S. Our MC-CDMA is NOT the same as DS-CDMA using multiple carriers.

WHAT ARE THE ADVANTAGES OF MC-CDMA?

Compared to Direct Sequence (DS) CDMA.
DS-CDMA is a method to share spectrum among multiple simultaneous users. Moreover, it can exploit frequency diversity, using RAKE receivers. However, in a dispersive multipath channel, DS-CDMA with a spread factor N can accommodate N simultaneous users only if highly complex interference cancellation techniques are used. In practice this is difficult to implement. MC-CDMA can handle N simultaneous users with good BER, using standard receiver techniques.
Compared to OFDM.
To avoid excessive bit errors on subcarriers that are in a deep fade, OFDM typically applies coding. Hence, the number of subcarriers needed is larger than the number of bits or symbols transmitted simultaneously. MC-CDMA replaces this encoder by an NxN matrix operation. See: Derivation. More advanced derivation including Doppler

MC-CDMA Transmitter

Figure: possible implementation of an Multi-Carrier spread-spectrum transmitter. Each bit is transmitted over N different subcarriers. Each subcarrier has its own phase offset, determined by the spreading code.

MC-Code Division Multiple Access systems allow simultaneous transmission of several such user signals on the same set of subcarriers. In the downlink multiplexer, this can be implemented using an Inverse FFT and a Code Matrix.

Figure: FFT implementation of an MC-CDMA base station multiplexer and transmitter.

MC-CDMA as a special case of DS-CDMA

Figure: possible implementation of a Multi-Carrier spread-spectrum transmitter. Each bit is transmitted over N different subcarriers. Each subcarrier has its own phase offset, determined by the spreading code. Note that the code is fixed over time, but only varies with subcarrier frequency!

The above transmitter can also be implemented as a Direct-Sequence CDMA transmitter, i.e., one in which the user signal is multiplied by a fast code sequence. However, the new code sequence is the Discrete Fourier Transform of a binary, say, Walsh Hadamard code sequence, so it has complex values.

Figure: Alternative implementation of a Multi-Carrier spread-spectrum transmitter, using the Direct sequence principle.

SOME PUBLICATIONS

N.Yee, J.P.M.G. Linnartz and G. Fettweis, Multi-Carrier-CDMA in indoor wireless networks, Conference Proceedings PIMRC '93, Yokohama, Sept, 1993. p 109-113 (scanned paper pdf of 2.8 mbytes Linnartz MC-CDMA PIMRC 1993)
This paper presented our first ideas on MC-CDMA. It is the first paper on this form of modulation, and the first to use the name MC-CDMA.
N.Yee, J.P.M.G. Linnartz and G. Fettweis, Multi-Carrier-CDMA in indoor wireless networks, IEICE Transaction on Communications, Japan, Vol. E77-B, No. 7, July 1994, pp. 900- 904.
N. Yee and J.P.M.G. Linnartz, "Controlled equalization for Multi-Carrier CDMA" VTC 1994, Stockholm, June 1994.PDF

An improved receiver method to MC-CDMA. It addresses the question of how to weight different subcarriers with different channel attenuation. On one hand, Maximum Ratio Combining is optimum against noise, but gives poor performance if multiple MC-CDMA signals are present. On the other hand, equalizing the amplitudes of all subcarriers in a downlink restores the orthogonality, but excessively boosts the noise
N. Yee and J.P.M.G. Linnartz, "Wiener filtering for Multi-Carrier CDMA", IEEE / ICCC conference on Personal Indoor Mobile Radio Communications (PIMRC) and Wireless Computer Networks (WCN), The Hague, September 19-23, 1994, Vol. 4, pp. 1344-1347.

This paper finds optimum weight (MMSE) factor for subcarriers. It shows that with a fairly straigtforward receiver implementation, good BER can be achieved (10^-5 at 16 dB SNR), even if N users are present in an MC-CDMA system with spread factor N. Performance improves with increasing (!) channel dispersion and with increasing N.
J.P.M.G. Linnartz and A. Gorokhov, "New equalization approach for OFDM over dispersive and rapidly time varying channel.", PIMRC ’00, London, Sept. 2000
J.P.M.G. Linnartz, "Synchronous MC-CDMA in dispersive, mobile Rayleigh channels", IEEE Benelux Signal Processing Symposium, SPS200, Hilvarenbeek, March 23-24, 2000.
Studies the effect of Doppler on OFDM and MC-CDMA.
N. Yee and J.P.M.G. Linnartz, "Multi-Carrier in an indoor wireless radio channel", Memorandum UCB/ERL M94/6, U.C. Berkeley, 1994.
[94-R1] N. Yee and J.P.M.G. Linnartz, "Multi-Carrier in an indoor wireless radio channel", Memorandum UCB/ERL M94/6, U.C. Berkeley, 1994. http://www.eecs.berkeley.edu/Pubs/TechRpts/1994/ERL-94-6.pdf (UCB bib data)
N. Yee and J.P.M.G. Linnartz, "Multi-Carrier Mc-CDMA Receiver Code Division Multiple Access (MC-CDMA): a new spreading technique for communication over multipath channels", Final Report 93-101 (also 92-092), Microelectronics Innovation and Computer Research Opportunities, University of California, Oakland, CA 94612-3550.
J.P.M.G. Linnartz, "Performance Analysis of Synchronous MC-CDMA in mobile Rayleigh channels with both Delay and Doppler spreads", IEEE Transactions on VT, Nov. 2001.PDF
Wireless Communication, The Interactive Multimedia CD ROM.
An on-line tutorial on OFDM and MC-CDMA (SMIL / Real-Audio).
OFDM in mobile communications; The effect of Doppler
Profs. Linnartz and Hara are guest editors of a special issue on Multi-Carrier Communications for "Wireless Personal Communications" (Kluwer).
300k PDF [01-J2] PDF J.P.M.G. Linnartz, "Performance Analysis of Synchronous MC-CDMA in mobile Rayleigh channels with both Delay and Doppler spreads", IEEE VT, Vol. 50, No. 6, Nov. 2001, pp 1375-1387.